Can I instal Ubuntu on an XP machine with 74GB I don't need any existing files or programs

Sure, Ubuntu only needs about 8Gb for the full OS including CD and DVD burning, office productivity and web apps. You can even resize the NTFS partition if you wish and make a dual boot.

It also depends on the amount of internal RAM in your PC. How much RAM memory does it have? And what is the model and make of the PC?

ubuntu works fine but for best performance on older PC's, use XUBUNTU or LUBUNTU. both are official derivatives of ubuntu and are really fast on older hardware. one GB ram is recommended for running unity in ubuntu where as xubuntu and lubuntu work fine even on pc with 256mb ram.

The system I have is XP 2002 SP2; Celeron 2.93GHz;504 GB RAM. I will need a linux program that is pretty much self installing since I am not at all good at using the DOS system to install/modify programs. I think I will need a program that installs from a CD. Should I format the C drive before I install Linux? I think I know how to do that.

Grab the Ubuntu install CD and boot to it, you can delete XP and install the OS on it's own. The installer can be told to use the entire drive space and xP will be removed for you